1. Field of the Invention
In one aspect, this invention relates to a fractionating process which produces a distillate having improved product quality. In another aspect, this invention relates to a method to increase fractionator capacity. In still another aspect, this invention relates to a method of increasing heat removal from a fractionator. In an additional aspect, this invention relates to an improved fractionator.
2. Background
Complex fractionation is used in petroleum refining to separate fractionator feed mixtures, such as crude oil, reactor effluent, coker gas oils, and other heavy oil streams, into various distillate, intermediate product and residual streams. The overhead distillate and column intermediate sidestreams typically include a wide-boiling range of materials and the residual liquid from the column bottoms is generally a relatively high boiling residuum.
Heavy oil fractionating processes generally employ somewhat different fractionator designs and processing methods than light hydrocarbon fractionation, although there are many process and equipment similarities. For instance, reboilers are generally used in light hydrocarbon fractionation; however, reboilers are not generally used in crude distillation because the feed is commonly heated to its maximum allowable temperature in a crude furnace prior to being fed to the crude tower. Heavy oil fractionator design and operation thus entail special considerations.
Common to both heavy oil and light hydrocarbon fractionation is a fractionating tower which contains a number of trays where ascending vapor in the tower is contacted with liquid cascading down the tower, which contact causes heat and mass interchange on each tray. Each tray comprises a downcomer to direct liquid from the tray to the tray below, a weir to maintain a liquid level on the tray, perforations to permit vapor passage upward through the tray, and a contacting means such as bubble caps, tray valves, or the like to enhance liquid-vapor contact. The vapor becomes progressively lighter as it rises through the tower and the liquid becomes progressively heavier as it passes down the tower. The vapor leaving the tower is totally or partially condensed and generally at least a portion of the condensed stream is returned back to the top tray of the tower as reflux and a portion is recovered as distillate product.
It is generally desirable to produce a distillate product with a relatively higher concentration of more volatile, lower boiling components of the feed. Such distillate is obtained by increasing reflux and the number of trays in the fractionator. The concentration of light components in the distillate is increased as the amount of reflux returned to the fractionator is increased as measured by the reflux ratio. The reflux ratio is indicated either (i) as the liquid-to-vapor ratio which is the molal rate at which liquid flows through the column divided by the molal rate at which vapor flows through the column or (ii) as the liquid-to-distillate ratio which is molal rate at which reflux is returned to the column divided by the molal rate at which distillate is withdrawn from the column.
At a given feed rate, distillate rate and bottoms product rate, the separation in the fractionator is improved as the reflux rate to the top tray is increased; however, condenser duty increases and furnace and/or reboiler duty increases as the reflux rate is increased. Also, as the reflux rate is increased, the diameter of the fractionating column must be increased in order to accommodate large vapor and liquid loads in the column. Thus as a practical and economic matter, as reflux to the top tray is increased, both the cost of the construction and the cost of operation of the fractionating column increase.
An alternative method for generating column reflux that is widely practiced in crude and other heavy oil fractionators is called pumparound heat removal or pumparound reflux. Although ample column reflux could be achieved by reflux to the top tray of the fractionator, pumparound reflux provides for heat removal in addition to the overhead condenser and permits improved thermal efficiency and reductions in column diameter and related construction and operating costs. In pumparound reflux, a "partial draw tray" or a "total draw tray" arrangement may be utilized. Partial draw trays are commonly utilized in crude towers and total draw trays are commonly utilized in the wash oil sections of vacuum towers, coker fractionators and other heavier oil fractionators.
In a typical equipment arrangement for pumparound reflux utilizing a "partial draw tray", as such term is used in the specification and claims, part of the liquid on the draw tray is permitted to flow through the draw tray downcomer to the tray below and part of the draw tray liquid is withdrawn from the draw tray and the withdrawn stream is routed to a pump and the pump discharge is separated into two portions. One portion is withdrawn as a side product. The second portion, the pumpup, is cooled, often by heat exchange to preheat the feed to the fractionator, and the cooled pumpup is recycled to the tower to a return tray which is usually one or two trays above the draw tray.
In a typical equipment arrangement for pumparound reflux utilizing a "total draw tray", as such term is used in the specification and claims, the fractionator comprises a draw tray which has a sealed downcomer or has no downcomer or weir or other means to pass liquid to the tray below and reflux to the tray below the draw tray must be supplied externally; however, as with other, non-draw, trays, the draw tray comprises perforations to permit vapor flow upward through the draw tray from the tray below the draw tray, and further comprises contact means to enhance liquid-vapor contact, such as bubble caps, valves or other contactors. Typically, the fractionator interior walls immediately above the draw tray provide the boundary against which liquid level builds on the draw tray. In a typical application of pumparound reflux utilizing a total draw tray, all or part of the liquid retained on the draw tray is withdrawn from the column and the withdrawn stream is routed to a pump and the pump discharge is separated into three portions. One portion is withdrawn as a side product. The second portion, the pumpup, is cooled, often by heat exchange to preheat the feed to the fractionator. The cooled pumpup is recycled to the tower to a return tray which is usually one or two trays above the draw tray. The third portion, the pumpdown, is recycled to the column below the draw tray.
Pumparound reflux reduces column liquid-vapor flow and reduces column diameter and related capital investment and operating costs; however, pumparound reflux has a capacity disadvantage in that the three pumparound zone trays, including the draw tray, the return tray, and the tray intermediate between the draw and return trays provide the separation efficiency only of one theoretical tray for fractionation purposes since the pumparound zone where the liquid from the draw tray is recycled to the return tray is a zone of constant composition and mass transfer is penalized, and capacity and operating efficiency of the fractionator are thereby impacted.
In addition, often in order to maximize the amount of separation zone heat removal, the rate of liquid pumpup to the tower is increased to an amount at which flooding of the tower occurs. When the rate of liquid flows down the column, which liquid flows include the liquid load added by the pumpup stream, are increased in any one tray section or zone comprising several trays, limits are reached wherein the downcomers fill with liquid and a condition of liquid flooding occurs. A flooded tray zone, which may include two or more flooded trays, gives a separation equivalent to one theoretical plate and column separation efficiency is thus reduced.
It is desirable to have an improved fractionating process providing for increased heat removal from the fractionator and a fractionating column which has increased capacity.